VITAL-DEP (Depression Endpoint Prevention in the VITamin D and OmegA-3 TriaL) is an ongoing, large- scale, 2x2 factorial randomized trial of vitamin D (vitamin D3 [cholecalciferol] 2000 IU/d) and omega-3 fatty acids (EPA and DHA, in 1:1 ratio, 1000 mg/d) for the prevention of late-life depression and promotion of long- term benefits for mood. In the first funding period (9/29/10-06/30/15), we completed baseline and early follow- up of depression outcomes among nearly 19,000 successfully randomized men and women, aged 50+ years (mean age=65y), who were eligible for incidence or recurrence of late-life depression; nearly 70% of participants have provided pre-randomization blood samples for nutrient biochemical assays. Furthermore, in the Clinical and Translational Science Center (CTSC) component, detailed phenotypic assessments were conducted among a subset of over 1,000 individuals who received in-person structured psychiatric diagnostic interviews, underwent neuropsychological testing, and provided data on dimensional measures of mood, well- being, and social and daily functioning. Finally, another landmark aspect of VITAL-DEP is its substantial racial and ethnic diversity: over 25% of participants are from minority racial and/or ethnic backgrounds, including 19% who are African American. Indeed, our first funding period aims emphasized potential differences in vitamin D response among African Americans, given high risk of low 25(OH)D in this group. However, late- breaking evidence indicates that bioavailable vitamin D is of greatest relevance to race/ethnic differences. In this 5-year renewal proposal, we expand upon what has been achieved in the first period - assembly of a unique trial cohort - and focus on the unprecedented scientific opportunities afforded by this RCT design. Our Primary Aims are: 1) to extend follow-up of mood to examine with high statistical power the long-term (up to 7 years) effects of vitamin D and marine omega-3 fatty acids on depression outcomes among all ~19,000 participants; 2) to examine with high power the impact of biochemical nutrients levels of vitamin D and omega- 3 fatty acids on depression outcomes as well as treatment interaction with baseline biochemical levels. Secondary Aims focus on two critical themes that our particular trial design is uniquely poised to address: 1) racial/ethnic differences in long-term treatment effects; 2) mechanisms, mediators and moderators of mood effects of vitamin D and fish oil in this diverse cohort. Specifically, in a sub-set of 2,000 participants, we will examine relations of bioavailable vitamin D to depression, with attention to race differences in outcomes. Further, among the entire eligible CTSC cohort, we will obtain pre- and post-randomization serum brain- derived neurotrophic factor and targeted plasma metabolite levels to examine biological paths involved in mood responses to vitamin D and fish oil treatment and to test whether biomarker change explains variation in treatment effects. Thus, this proposal not only will ensure comprehensive, definitive testing of the benefits of vitamin D and fish oil for late-life mood but also will inform specific mechanisms involved in a diverse sample.